If your files go to "need more sourses" you need to manually restart them. Sometimes they go to "need more sourses for the reason that possibly there was a short break in the connection, and this can be very annoying. Usually, though it is because the host has gone off line. The reason that files that "need more sourses" do not search automatically is because if they did the Gnutella network would be flooded to cracking point.
If you have a file in the download window "awaiting sourses" I THINK in theory that Limewire should recommence download as soon as the file's host(s) come back on line. In practice this does not happen quite as often as it should, and often a successful fresh search for a file, if it is in your download list, seems to be likely to trigger the file into downloading.
I'm not sure if this exactly answers your question, so let me know if it's no clearer and I'll try again
If your preferences are set to clear downloads automatically that makes no difference. That just refers to the visual display in the download window. I do not set mine to clear automatically as I like to check everything I have downloaded is where it should be before clearing the list.